Summary

Disposal of produced water from oil fields is a major concern forenvironmental and economic reasons.One way to dispose of this water is tomix it with aquifer water and inject it back into the formation.One ofthe carbonate reservoirs in Saudi Arabia produces wet oil where the totaldissolved solids (TDS) of the produced water is high (up to 238,000 mg/L). Theproduced water also contains dissolved gasses (H2S and CO2) and suspendedmaterial (oil and corrosion products). Thus, disposal water from a gas/oilseparation plant (GOSP) not only has CaCO3 scaling potential, but can alsoprecipitate sulfides because it contains nearly 750 mg/L of H2S. Incontrast, the aquifer waters from the area contain approximately 2 mg/L oftotal iron. Therefore, if these two waters were to be mixed, the possibility ofdamaging the formation from iron sulfide precipitation could be signficant. Theobjective of this study is to assess potential formation damage that can resultwhen the two waters are mixed and injected into the tight carbonatereservoir.

The current study included a detailed analysis of field waters,determination of scaling potential of various waters, and extensive corefloodtesting using reservoir cores. A unique feature of this study was that thecores were examined after the injection of the mixed waters by computerizedtomography (CT) and scanning electron microscopy (SEM) to determine both macroand pore-level effects on core properties.

The study revealed that the injection of mixed water into reservoir corescreated wormholes, which increased core permeability. This new findingindicates that disposal water is not always damaging. On the other hand, theprecipitated iron sulfide particles seem to damage the cores, but the damagewas constrained by the core initial permeability. For example, the iron sulfideparticles (0.25 GREEK mu m) caused damage to cores with permeability of lessthan 20 md after injecting 1,000 pore volumes (PV) of the mixed waters.However, no damage was observed in cores with permeability greater than 60 md,even after injecting 600 PV of the same mixed water.

The study identified various types of scale resulting from mixing producedand aquifer waters, and determined conditions under which these waters could beinjected. Also, the study highlights an unexpected benefit of injectingH2S-containing waters, which is to create wormholes and thus significantlyincrease permeability of tight carbonate reservoirs.

Introduction and Background

Produced water reinjection (PWRI) has been carried out throughout the worldto minimize environmental concerns. However, there are only a few studies onthe effects of commingled produced and injection water (sea or aquifer water),and still fewer numbers of actual field cases. Incompatibilities and otheroperational problems are the main concern, although economics favor themixed-water injection.